According to a conventional method, the lowest value of energy consumption is calculated using the mixed integer programming for each case in which the models and the number of computers concurrently operated differs; the case having the lowest energy consumption is ultimately selected among the cases; and thereby, plural energy supply units are operated to distribute the load thereto (see, e.g., Japanese Laid-Open Patent Publication No. H6-141468). According to another method, a production request is accepted that includes the number of processors, the memory amount, and an assignment policy for resources, of a virtual server that is to be produced; and processors and memories are assigned to the virtual server based on the accepted production request such that the assignment policy is satisfied (see, e.g., Japanese Laid-Open Patent Publication No. 2009-151745). According to yet another method, power consumption is calculated for each software process based on the amount of resources used when a processor executes the software process; some of the software processes are moved from a first processor to a second processor according to the calculated power consumption (see, e.g., Japanese Laid-Open Patent Publication No. 2010-205200). According to another method, the amount of resources to be assigned to a logical server is calculated for each combination of a logical server and physical server resources to minimize the power consumption of plural servers and that of plural air-conditioners; and the assignment of the resources of the physical server to the logical server is changed (see, e.g., Japanese Laid-Open Patent Publication No. 2011-39889). According to still another method, workloads are assigned to information processing apparatuses using the positions of and operation information concerning the information processing apparatuses, and positions of and environment information concerning power supply facility and cooling facility such that the total power consumption of the information processing apparatuses, the power supply loss of the power supply facility, and electric power for cooling used by the cooling facility is reduced (see, e.g., Japanese Laid-Open Patent Publication No. 2009-252056).
However, in a computer system that includes plural racks respectively having an uninterruptible power supply (UPS) and plural servers, according to the conventional methods, a problem arises in that plural virtual machines cannot be assigned to the plural servers. Even when plural virtual machines are assigned to the plural servers according to one of the conventional methods, another problem arises in that in some cases no electric power saving can be facilitated.